1. Field of the Invention
The present invention relates to a radiographic image detector on which a radiographic image is recorded by being irradiated with radiation carrying the radiographic image, and from which a signal corresponding to that radiographic image is read out by being scanned with image-reading light.
2. Description of the Related Art
In the medical field, a variety of radiographic image detectors have been proposed and put to practical use, in which electric charges are produced by being irradiated with radiation transmitted through a radiographer object, and a radiographic image relating to the radiographer object is recorded by storing the produced electric charges.
Such a radiographic image detector has been proposed, for example, in U.S. Pat. No. 6,770,901. This radiographic image detector includes a first electrode layer for transmitting radiation, a photoconductive recording layer for producing electric charges by being irradiated with radiation, an electric-charge transporting layer that operates as an insulator with respect to latent-image electric charges and operates as a conductor with respect to transport electric charges having polarities opposite from the latent-image electric charges, a photoconductive read-out layer for producing electric charges by being irradiated with image-reading light, and a second electrode layer in which first line electrodes for transmitting the image-reading light and second line electrodes for intercepting the image-reading light are alternately arranged in parallel. These layers are stacked in the recited order.
When recording a radiographic image onto the radiographic image detector constructed as described above, radiation is first irradiated onto the detector 20 with a negative high voltage applied to the first electrode layer 1 of the detector 20 by a high-voltage source 30 connected to the first electrode layer 1, as shown in FIG. 8A. Then, the radiation is transmitted through the first electrode layer 1 and irradiated onto a photoconductive recording layer 2. At a portion of the photoconductive recording layer 2 irradiated with radiation, electric charge pairs are produced. The positive electric charges of the electric charge pairs move toward the negative-charged first electrode layer 1, combine with the negative electric charges in the first electrode layer 1 and are neutralized. On the other hand, the negative charges of the electric charge pairs move toward a positive-charged second electrode layer 5, but since an electric-charge transporting layer 3 operates as an insulator with respect to negative charges, the above-described negative charges are stored at an charge accumulating portion 6, which is the interface between the photoconductive recording layer 2 and the electric-charge transporting layer 3. A radiographic image is recorded by the storage of the negative charges on the charge accumulating portion 6.
And when reading out from the radiographic image detector 20 the radiographic image recorded as described above, image-reading light is first irradiated onto the second electrode layer 5, as shown in FIG. 9. The image-reading light is transmitted through the first line electrode 5a of the second electrode layer 5 and is irradiated onto a photoconductive read-out layer 4, in which electric charge pairs are produced. The positive charges of the produced electric charge pairs combine with the negative charges stored at the charge accumulating portion 6, while the negative charges of the electric charge pairs combine with positive charges at the first line electrode 5a and with positive charges flowing to the first line electrode 5a through ground from a second line electrode 5b. As a result, a current I is detected by a current-detecting amplifier 8 connected to the first line electrode 5a. The current I is converted into a voltage, which is output as an image signal.
However, when image-reading light is irradiated near the edge of the second line electrode 5b, as shown in FIG. 9, there are cases where electric discharge occurs near the edge of the second line electrode 5b. Positive charges produced by this edge discharge combine with the negative charges stored at the charge accumulating portion, so the amount of negative charges to combine with the positive charges at the first line electrode 5a is reduced. For this reason, the current I detected by the current-detecting amplifier is reduced and the image read-out efficiency is reduced. This results in the degradation of the picture quality of a read out radiographic image.